High density plasma oxidation

ABSTRACT

A method of oxidizing a substrate having area of about 30,000 mm 2  or more. The surface is preferably comprised of silicon-containing materials, such as silicon, silicon germanium, silicon carbide, silicon nitride, and metal silicides. A mixture of oxygen-bearing gas and diluent gas normally non-reactive to oxygen, such as Ne, Ar, Kr, Xe, and/or Rn are ionized to create a plasma having an electron density of at least about 1 e12 cm −3  and containing ambient electrons having an average temperature greater than about 1 eV. The substrate surface is oxidized with energetic particles, comprising primarily atomic oxygen, created in the plasma to form an oxide film of substantially uniform thickness. The oxidation of the substrate takes place at a temperature below about 700° C., e.g., between about room temperature, 20° C., and about 500° C.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and system for oxidizingsurfaces and, in particular, is directed to oxidizing silicon-containingand other semiconductor wafers in a high-density plasma chemical vapordeposition tool.

2. Description of Related Art

In general, an oxidation process is performed by heating wafers in anambient of oxygen-bearing gas. Typical oxygen-bearing gases aremolecular oxygen O₂, water vapors H₂O, nitrous oxide N₂O, nitric oxideNO, and ozone O₃. Oxidation reactors can be roughly divided into singlewafer tools and furnaces. A typical oxidation furnace processes manywafers at once. Modern oxidation furnaces can process 100-200200-mm-diameter wafers in one batch. Due to the large batch size, thefurnaces can support relatively slow processes without affectingthroughput. A typical furnace throughput is 50-100 wafers per hour. Onthe other hand, single wafer tools process one wafer at a time. Singlewafer tools allow for a greater manufacturing flexibility, for they cansupport a different process per given wafer. A manufacturing facilitythat processes a large mix of different products can greatly benefitfrom the flexibility of single wafer oxidation tools. In order tosupport a competitive throughput as compared to that of a furnace,single wafer tools employ fast oxidation processes. A typical throughputof a single wafer oxidation tool is 20-40 wafers per hour. Single waferoxidation processes longer than 5 minutes are highly undesirable. Infact, a preferred single wafer process is less than 2 minute long.

General trends in micro- and nanofabrication are directed to thereduction of thermal budget, processing of large substrates, andincreased three-dimensional integration. Reduced thermal budget allowsfor sharp dopant profiles and prevents chemical reaction and intermixingbetween adjacent dissimilar materials. Consequently, the device featurescan be made smaller without loss of structural integrity. Largersubstrates allow for more devices to be manufactured per givenprocessing sequence. As the result, the manufacturing cost per givendevice is reduced. Integration of various devices into the thirddimension (perpendicular to the wafer surface) offers even greaterdevice density.

One way to speed up the oxidation process is to use a highly reactiveoxygen-bearing gas that can rapidly react with the substrate at a lowtemperature. The most reactive oxygen-bearing gas is free oxygen radicalor atomic oxygen. In order to produce a substantial amount of atomicoxygen, one has to dissociate more stable oxygen-bearing gas (e.g. O₂,O₃, NO, N₂O) with the aid of some excitation. The excitation can be inthe form of electrical discharge, flux of photons (photo assisted),electron beam, or localized intense heat. The atomic oxygen isinherently unstable substance and may quickly recombine without reachingthe substrate. Furthermore, providing a uniform distribution of atomicoxygen over relatively large substrate (200-mm in diameter and larger)is a challenge. Accordingly, the atomic oxygen has to be first producedwith the aid of some excitation, than delivered to the substrate withminimal recombination losses, and finally redistributed over thesubstrate surface in a manner that ensures an acceptable uniformity ofthe process.

There are several known processes and tools that employ atomic oxygenfor a fast, low temperature oxidation process.

A low temperature, charge-free process for forming oxide layers wasdisclosed in U.S. Pat. No. 4,474,829, which utilized anoxygen-containing precursor and exposed it to radiation of a selectedwavelength to cause direct dissociation to generate oxygen solely inatomic form. However, this process is relatively complex and requiresspecially-built tools, not readily commercially available tools such asan RTO oxidation tool.

Gronet et al. U.S. Pat. No. 6,037,273 discloses an apparatus to carryout an in-situ steam generation (ISSG) oxidation technique. Gronetdiscloses that the in-situ steam generation rapid thermal processor (asingle wafer tool) is well suited for high volume semiconductormanufacturing due to a superior temperature uniformity, fast temperatureramps, high throughput, and acceptable safety record. Gronet disclosesthat a substrate can be placed in such a reactor and then oxidized usingthe in-situ generated steam. Gronet discloses a fast oxidation of asubstrate having a Si layer. Tews et al. U.S. Pat. No. 6,358,867 teachesthat the oxidation process conducted in in-situ steam generation rapidthermal processor shows little orientation dependence. Tews et al.teaches that the absence of orientation dependence is the earmark ofatomic oxidation. Tews et al. refers to the ISSG oxidation technique asfree radical enhanced rapid thermal oxidation (FRE RTO), In a relatedapplication, Ser. No. 09/874,144, Ballantine et al. teaches that theISSG process is capable of rapidly oxidizing very stable siliconnitride. Ballantine et al. teaches that the rapid oxidation of SiN canbe only performed with the aid of some excitation.

Oxidation by use of plasmas has also been disclosed in U.S. Pat. Nos.4,232,057, 4,323,589, 5,330,935, 5,443,863, 5,872,052, 5,913,149,5,923,948 and 6,165,834. While the disclosed plasma oxidation methodshave shown some promise, they have resulted in relatively low oxidegrowth-rate, have been limited to relatively small substrate surfaceareas or low density plasma (which gives poor throughput for thickerfilms) or have had other problems which have made them unable to competewith the commercially available RTO oxidation tools.

FRE RTO or ISSG process can provide substantial amount of atomic oxygenover 200-mm substrate resulting in a number of useful atomic oxidationprocesses such as orientation independent oxidation of silicon and fastoxidation of silicon nitride. Nevertheless, the process is limited thehigh substrate temperature (above 600 C) because the atomic oxygen isgenerated within the chamber as a secondary byproduct of multi-stepreaction between hydrogen H2 and oxygen O2. Furthermore, safetyrequirements limit the process substrate temperature to even highertemperature (above 800 C). This temperature range is relatively highcompare to the atomic oxidation processes where the radicals are createdin a plasma discharge.

SUMMARY OF THE INVENTION

Bearing in mind the problems and deficiencies of the prior art, it istherefore an object of the present invention to provide a method andsystem for oxidizing substrates which has relatively high oxide growthrate and utilizes relatively inexpensive reactants.

It is another object of the present invention to provide a method andsystem for oxidizing substrates which utilize readily available toolsutilized in the semiconductor industry.

A further object of the invention is to provide a method and system foroxidizing a semiconductor substrate which may be used for relativelylarge substrates, for example wafers of at least 200 mm in diameter.

It is yet another object of the present invention to provide a methodand system for oxidizing a semiconductor substrate which results in ahigh degree of oxide uniformity across the surface.

It is yet another object of the present invention to provide a methodfor oxidizing a semiconductor substrate which may be performed atrelatively low temperatures.

The above and other objects and advantages, which will be apparent toone of skill in the art, are achieved in the present invention which isdirected to, in a first aspect, a method of oxidizing a substrate usinga plasma comprising providing a large-area substrate with area of about30,000 mm² or more having a surface capable of being converted to anoxide and providing a mixture of oxygen-bearing gas and diluent gasnormally non-reactive to oxygen. The oxygen and diluent gas mixture areionized to create a plasma having an electron density of at least about1 e12 cm⁻³ and the substrate surface is oxidized with energeticparticles created in the plasma to form an oxide film of substantiallyuniform thickness.

The oxygen gas may comprise between about 10% and 95% of the mixture bymole fraction and the diluent gas may comprise between about 90% and 5%of the mixture by mole fraction and may be Ne, Ar, Kr, Xe, and/or Rn.The energetic particles comprise primarily atomic oxygen and the plasmacontains ambient electrons having an average temperature greater thanabout 1 eV. Preferably, the oxidation of the substrate takes place at atemperature below about 700° C., and more preferably takes place at atemperature between about room temperature, 20° C., and about 500° C.

While the substrate can be comprised of various materials and cancontain various structures, preferably, the substrate surface iscomprised of a silicon-containing material, such as silicon, silicongermanium, silicon carbide, silicon nitride, and/or metal silicides. Thepresent invention provides an additional advantage when the substratesurface is comprised of dissimilar materials. The dissimilar materialsmay include monocrystalline silicon with different crystallographicplanes, particularly, (100), (110) or (111) planes. The dissimilarproperty of material can also be due to a different level of doping ordoping type. According to present invention, an oxide film grown ondissimilar materials is of substantially the same thickness. The oxidethickness variation from material to material is less than 20% while theoxide thickness variation over entire substrate surface made of one typeof material is less than 2% at 1σ standard deviation.

In another aspect, the present invention is directed to a method ofoxidizing a substrate using high density plasma comprising providing ahigh density plasma reactor, placing a substrate with an area of atleast about 30,000 mm² and having a surface capable of being convertedto an oxide in the reactor, and introducing a mixture of oxygen gas anda diluent gas normally non-reactive to oxygen into the reactor. Theoxygen and diluent gas mixture is ionized to create a plasma having anelectron density of at least about 1 e12 cm⁻³ and the substrate surfaceis oxidized with energetic particles created in the plasma to form anoxide film with the thickness range of 20% or less of the targetthickness.

In a further aspect, the present invention provides a high densityplasma reactor system for oxidizing a substrate comprising a containmentstructure for creating and maintaining a plasma and a substrate with anarea of at least about 30,000 mm² and having a surface capable of beingconverted to an oxide within the containment structure. The systemincludes a plasma, over the surface of the substrate, created from amixture of oxygen gas and a diluent gas normally non-reactive to oxygen,the plasma having an electron density of at least about 1 e12 cm⁻³, andbeing capable of oxidizing the substrate surface with energeticparticles created in the plasma to form an oxide film with the thicknessrange of 20% or less of the target thickness.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention believed to be novel and the elementscharacteristic of the invention are set forth with particularity in theappended claims. The figures are for illustration purposes only and arenot drawn to scale. The invention itself, however, both as toorganization and method of operation, may best be understood byreference to the detailed description which follows taken in conjunctionwith the accompanying drawings in which:

FIG. 1 is a side elevational view, in cross section, of a high-densityplasma containment vessel or reactor which is useful in practicing thepresent invention.

FIG. 2 is graphical representation of a comparison of the oxide growthrate of the present invention on three different substrates, as comparedto a prior art oxidation method.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

In describing the preferred embodiment of the present invention,reference will be made herein to FIGS. 1 and 2 of the drawings in whichlike numerals refer to like features of the invention. Features of theinvention are not necessarily shown to scale in the drawings.

The present invention provides a method and system for oxidizingsemiconductor substrates which utilizes a readily commercially availablehigh density plasma reactor of the type normally used for chemical vapordeposition (CVD). The substrate or wafer can be comprised of eitherconductors (e.g., elemental metals, metal silicides, certain metalnitrides, and the like), semiconductors (e.g., Si, Ge, SiGe, SiC, SiGeC,or any of III-V or II-VI compounds), insulators and insulating ceramics(e.g. glass, sapphire, silicon nitride, boron nitride, etc.), andpolymers. The substrate can have multiple layers of these materials(e.g. silicon-on-insulator (SOI) and silicon-germanium-on-insulator(SGOI) substrates, strained silicon substrates and others). Materials onthe substrate surface can be in polycrystalline, monocrystalline, oramorphous form, doped or undoped. The substrate can contain varioususeful structures such as isolation structures including isolationtrenches, differently doped areas including transistor wells, capacitorsincluding trench and stack capacitors, transistors, interconnects,optical devices, micro electromechanical systems (MEMS), and otherstructures. The substrate structures can have planar orthree-dimensional geometry. The preferred substrate surface includessilicon-containing materials such as such as silicon, silicon germanium,silicon carbide, silicon nitride, and/or metal silicides. The methodutilizes a relatively low temperature budget oxidation and results in anoxidation rate similar to that of commercial RTO oxidation tools.

As shown in FIG. 1, a standard CVD high density plasma tool comprises areactor 10 having a containment structure 30 which encloses a vacuumchamber 12 therewithin. The wafer or substrate 16 to be oxidized sitsupon a support which is also a radio frequency (RF) electrode 14,connected to an RF source 24. The presence of an independent substratebias is optional. The wafer support stage may also include a waferheater with its power supply, wafer temperature sensor, and controllingcircuitry (not shown). The surface of the wafer 16 to be oxidized isnormally facing upward into the interior of the chamber 12. A gas inlet22 is connected to a gas manifold 20 extending around chamber 12, whichmanifold is further connected to a plurality of nozzles 18 that directgas from the manifold into the chamber 12. A number of RF coils 32connected to an RF source 26 extend around the generally hemisphericaltop of reactor 10. The RF source 26 is typically connected to coils 32via an impedance matching network (not shown). The plasma is created inthe vicinity of coils 32. The plasma zone can be optionally separatedfrom the substrate by dividing the chamber with a plate containing aplurality of holes (not shown). The plasma is then confined in the upperportion of the reactor. The energetic particles created in the plasmaeffuse through the plate holes, reach the substrate surface, and reactwith it. Such plate is often called showerhead. Showerhead holedimension can vary to offset varying gas conductance over a largesubstrate thus improving process uniformity. A Faraday shield (notshown) may be inserted in between the coils 32 and the plasma tominimize capacitive coupling from the coil. The capacitive coupling mayresult into chamber or wafer damage associated with high-energy ions.

The substrate whose surface may be oxidized in accordance with thepresent invention may be any material which has a surface which may beconverted to a thermal oxide. The present invention is particularlysuitable for silicon-containing materials such as silicon, silicongermanium, silicon carbide, silicon nitride, and/or metal silicides. Inthe case of silicon crystal, its surface to be converted to an oxide canhave multiple crystallographic planes, for example (100), (110), or(111) planes. In accordance with the present invention, the thermaloxide grown on these planes is essentially of the same thickness, thushaving an orientation independent oxidation property. Furthermore, inaccordance with the present invention, the speed of oxidation of siliconnitride is similar to that of crystalline silicon showing that theinventive method oxidizes widely dissimilar materials with essentiallythe same speed. This universally isotropic oxidation property of theinventive method allows for growing a relatively uniform thermal oxideon different materials. For instance, a uniform layer of silicon oxidecan be grown over a three dimensional structure with its surfacecomprised of different crystallographic planes of silicon crystal andsilicon nitride isolation layers. The substrate is normally present inthe form of a circular wafer, and the present invention has been foundto be particularly useful where the wafer diameter is on the order of200 mm or more, providing a total area of at least about 30,000 mm².

The method of the present invention utilizes an oxygen-bearing gas suchas molecular oxygen (O₂) gas, along with a diluent gas. Preferably, thediluent gas is an inert gas, and is more preferably a heavy-moleculenoble gas such as argon. The mixture of the oxygen and diluent gaspreferably comprises about 10% and 95% by (mole fraction) of oxygen andbetween about 90% and 5% by (mole fraction) of the diluent gas. Themixture is then excited in a high-density (electron concentration ofabout 1 e12 cm⁻³ or more) electrical discharge to yield oxygen radicals.The oxygen radicals support the oxidation process. Their concentrationand its uniformity near the wafer surface affect the speed anduniformity of the oxidation process, respectively. It has beendiscovered that a 100% oxygen atmosphere in the plasma reactor does notprovide the desired high oxidation rate. In addition, it has been foundthat mixing a light noble gas such as helium He with oxygen does notresult in a substantial increase of the oxidation rate. Unexpectedly, ithas been found that a mixture of argon and oxygen results in arelatively high oxide growth rate that is at least about 50 Å/min asjudged by the final film thickness. Due to a non-linear (mostly squareroot) dependence of the oxide thickness on the oxidation time for atomicoxidation processes, it is more correctly to operate with a differentialgrowth rate (an increment of oxide film thickness per given oxidationperiod). The differential growth rate of 0.25-1 Å/sec for 50-150 Å thickfilms has been achieved. This rate is comparable to that of a typicalRTO process albeit at a much lower temperature. Typically, a mixture of75% (by mole fraction) of O2 and 25% (by mole fraction) of argon is mostdesirable, although a lower percentage of the argon, down to about 5% bymole fraction is believed useful. The preferred minimum amount of oxygenin the mixture is at least 20% (by mole fraction). Other heavy-moleculenoble gases such as Ne, Kr, Xe, and Rn are believed to result into asimilar increase of atomic oxygen generation in a high-density plasmas.

In practice, after the reactor 10 has been evacuated, the mixture ofoxygen and diluent gas is introduced via gas nozzles 18, the reactorpressure is maintained at a low pressure of from about 0.1 mTorr toabout 30 Torr, and plasma is created and maintained by theelectromagnetic energy supplied by means of inductive coupling betweenthe RF coils 32 and the ionized gas mixture. The coils 32 are driven bya radio frequency power supply via a matching network. The matchingnetwork minimizes electromagnetic power reflection by matchingelectrical impedances of coupled coil-plasma system and a power supplycable. The electromagnetic power is primarily deposited into the plasmaelectrons. The electrons then collide with other particles causingvarious electronic and vibrational excitation, ionization, anddissociation processes. The excited particles can collide causing newexcitation states and/or contributing to the existing energeticparticles. Subsequently, the electromagnetic power is divided amongnumerous excited particles. Only a relatively stable excited particlecan participate is the oxidation process that involves diffusion througha solid state film. While ions and molecules excited to variouselectronic states recombine upon a single collision with solid statesurface, oxygen radicals can survive many such encounters. While notwishing to be bound by theory, it is believed that atomic oxygen iscreated from the gas mixture to form energetic particles which cause theformation of thermal oxide on the surface of the substrate. Theproximity of the plasma to the wafers surface is not that important aslong as the atomic oxygen can diffuse from the plasma zone withoutsubstantial losses and without affecting its uniformity across the wafersurface. It is also believed that the plasma ambient within chamber 12contains atomic oxygen with the fraction of dissociation of higher than20%.

A preferred high plasma density i.e., a plasma having a number densityof electrons at about 1 e12 cm⁻³ or more, is related to the depositedpower density or at least 0.5 W/cm² or more. The deposited power densitycan be determined by dividing the deposited power by the reactor areacovered by coils 32. In turn, the deposited power can be determined fromthe power delivered to the matching network (the power setting of thereactor with negligible reflected power) multiplied by the powerefficiency of inductive coupling system. A typical power efficiency ofinductive coupling in such reactors is from 0.1 to 0.9, that is between10% and 90% of the power delivered to the matching network is actuallydeposited into the plasma while the rest is dissipated in the coils 32and the matching network. The oxidation was also performed at arelatively low temperature, i.e., less than about 700° C., and greaterthan about room temperature 20° C. The preferred temperature range isbetween about room temperature 20° C. and about 500° C., more preferablyless than 400° C., most preferably about 300° C. It has also been founduseful where the plasma ambient electrons have an average temperature ofhigher than 1 eV. As will be appreciated by those skilled in the plasmaart, these conditions may be affected by chamber geometry, particularplasma mixture, deposited power settings and other parameters. Theoptimum parameters may be determined without undue experimentation.

The oxidation reaction is carried on in the reactor until the desiredthickness of oxide is formed on the substrate surface. The presentinvention has been found to result in relatively fast oxide layerdifferential growth rates of 0.25-1 Å/sec. at approximately 300° C.reaction temperature. Such fast differential oxidation rate can supporta single wafer oxidation process of less than 5 minutes per wafer andtypically about 2 minutes per wafer while growing a relatively thickoxide film of 50-200 A. The present invention results in a relativelygood quality oxide, which may be useful for forming e-fuses, hard, masklayers, sidewall oxides, isolation oxides and the like on varioussilicon-containing materials often combined on the same substrate. Inaddition, other materials such metals, metal nitrides, metal oxides,compound semiconductors (typically III-V or II-VI semiconductor) andvarious polymers can be oxidized with this method at very lowtemperature to create useful oxide-based structures.

The present invention also results in a relatively high uniformity ofoxide thickness wherein the energetic particles created in the plasmaare capable of forming an oxide film with the thickness range of 20% orless of the target thickness on dissimilar materials, and 1σ standarddeviation of thickness variation less than about 2% over entire wafersurface of one material type.

A comparison of oxidation by the present invention with the prior art isdepicted in FIG. 2. The graph therein compares the time of oxidationreaction on the x-axis with the square of oxide thickness in Å² on they-axis. As can be seen, the rates of oxide formation for the presentinvention at approximately 350 C on silicon nitride, monocrystallinesilicon on the (100) and (110) crystalline faces compares favorably withthe rate for the prior art ISSG (FRE RTO) processes on monocrystallinesilicon on the (100) face at 900 C

Thus the present invention achieves the objects recited above andprovides a high oxidation rate at a relatively low temperature budgetutilizing existing and readily available high density plasma toolsnormally employed for chemical vapor deposition.

While the present invention has been particularly described, inconjunction with a specific preferred embodiment, it is evident thatmany alternatives, modifications and variations will be apparent tothose skilled in the art in light of the foregoing description. It istherefore contemplated that the appended claims will embrace any suchalternatives, modifications and variations as falling within the truescope and spirit of the present invention.

1-19. (canceled)
 20. A high density radio frequency plasma reactor system for oxidizing a substrate comprising: a containment structure for creating and maintaining a plasma; a substrate within the containment structure, the substrate having a substrate surface area comprised of dissimilar silicon-containing materials capable of being converted to an oxide and having a surface area of at least 30,000 mm²; and a plasma, over the surface of said substrate, created from a mixture of oxygen gas and a diluent gas normally non-reactive to oxygen, said plasma having an electron density of at least about 1 e12 cm⁻³ and being capable of oxidizing the substrate surface with energetic particles created in the plasma to form an oxide film of substantially uniform thickness such that oxide film thickness variation between the dissimilar materials is less than 20%.
 21. The system of claim 20 wherein said energetic particles comprise primarily atomic oxygen.
 22. The system of claim 20 wherein the oxygen gas comprises between 100% and 95% of the mixture by mole fraction.
 23. The system of claim 20 wherein the diluent gas comprises between 90% and 5% of the mixture by mole fraction.
 24. The system of claim 20 wherein the oxidation of the substrate takes place at a temperature below 700° C.
 25. The system of claim 20 wherein the oxidation of the substrate takes place at a temperature of between 20° C. and 500° C.
 26. The system of claim 20 wherein the plasma contains ambient electrons having an average temperature greater than 1 eV.
 27. The system of claim 20 wherein the dissimilar silicon-containing materials are selected from the group consisting of silicon, silicon germanium, silicon carbide, silicon nitride, and metal silicides.
 28. The system of claim 20 wherein the dissimilar silicon-containing materials are monocrystalline silicon having multiple crystallographic planes.
 29. The system of claim 28 wherein the multiple crystallographic planes include at least two of the following crystallographic planes: (100), (110), and (111).
 30. The system of claim 20 wherein the surface of the substrate converted to an oxide has a non-uniformity of less than 2% at 1σ standard deviation.
 31. The system of claim 20 wherein the oxygen gas comprises between 10% and 95% of the mixture by mole fraction and the diluent gas comprises between 90% and 5% of the mixture by mole fraction.
 32. The system of claim 20 wherein the plasma contains ambient electrons having an average temperature greater than 1 eV and said energetic particles comprise primarily atomic oxygen. 